FIG. 1 shows a block diagram of the basic operation of a conventional linear frequency-modulated (FM) pulse compression radar. The frequency of a pre-selected intermediate frequency (IF) signal pulse is made to vary linearly by any one of several well known methods. Frequency modulator 4 modulates this IF signal, increasing the frequency band with a carrier frequency large enough to effect efficient propagation of the pulse into space. The resulting FM signal pulse is a relatively long-duration, high-carrier-frequency pulse which is transmitted by transmitter 2 along a transmission line to a transmit-receive switch 8. Transmit-receive switch 8 then feeds the FM signal pulse to a directional antenna 10 which radiates the pulse in the form of a directional beam towards a target.
Return echoes of the transmitted FM signal pulses are received by antenna 10 and passed to a receiver 12 via transmit-receive switch 8. The return echo pulses are reduced down to an intermediate frequency when combined with local oscillator 16 in mixer 14. The echo pulses are then amplified by an IF amplifier 18 and directed into a pulse compression filter 20, e.g., a matched filter, which compresses the width of the echo pulses. The output of filter 20 is transformed into video pulses by detector 22 for display to a human operator on an indicator 26, e.g., a cathode ray tube. A linear FM pulse compression radar of this type is disclosed in U.S. Pat. No. 2,624,876.
It is well-known that the output of a matched filter is the autocorrelation function of its input signal. Accordingly, the time response of the output of a matched filter to a linear FM input signal appears as a well-defined narrow autocorrelation peak, or main lobe, surrounded by a plurality of temporal/range sidelobes. The main lobe represents the compressed pulse, while the temporal/range sidelobes can be interpreted as the value of the auto-correlation function at specific time lags. The amplitude of the temporal sidelobes of the compressed pulse signal output of a matched filter are smaller than the amplitude of the main lobe. For example, the peak sidelobe may be located approximately 13.5 dB down from the peak of the main lobe.
Target detection typically is achieved by comparing the amplitude of a compressed pulse signal with a predetermined threshold level. A false target is said to be present if the amplitude of any temporal sidelobe of the compressed pulse envelope exceeds this threshold level. Temporal sidelobes are especially susceptible to corruption.
Thus one of the limitations of conventional linear FM pulse compression radar is that the autocorrelation function output of a matched filter is contaminated with spurious responses which significantly compromise target detection performance.
More specifically, spurious responses in a pulse compression radar signal are objectionable for at least two reasons. First, they can produce false alarms during small-target detection. A false alarm occurs, for example, when the amplitude of any temporal sidelobe exceeds a predetermined threshold level. Second, spurious responses in a pulse compression radar signal can cause small-target suppression, as described in U.S. Pat. No. 4,507,659.
Techniques for reducing the spurious responses, and in turn, the amplitude of the temporal sidelobes of a linear FM pulse compression radar signal, are known. One common method used to reduce the amplitude of the temporal sidelobes is to use weighting functions, e.g., Taylor and Hamming weightings, in the frequency response of the matched filter. Weighting functions, however, reduce signal-to-noise ratio in the output signal, cause sensitivity losses in the matched filter, and consequently reduce the maximum range at which a given target can be detected. Weighting functions also increase the width of the compressed pulse, degrading the range resolution of the radar.
Non-linear pulse compression is another known technique for reducing the amplitude of temporal sidelobes. Successful implementation of this technique, however, requires accurate control of a number of waveform generating parameters, and therefore is more complex and costly than linear FM pulse compression.
Linear FM pulse compression radar is desirable because it permits the use of a long transmitter signal pulse while simultaneously realizing the benefits of large pulse energy and good range resolution of a short pulse. A need therefore exists to provide an effective method for improving the target detection performance of linear pulse compression radar without realizing the disadvantages of the aforementioned techniques.